George Michael Bluth's on again off again girlfriend, Ann Veal, is either disliked or ignored by the other members of the Bluth family (with the exception of when Gob dated her in the third season), but we never really learn why.

1 Answer
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Ann is made up to be rather plain and frumpy, so it's not so much that everyone else in the Bluth family dislikes her, they just don't see what George Michael sees in her. Michael in particular seems to think his son can do much better, and often gets Ann's name wrong, calling her Egg, Bland, Plant, and of course, "...Her?"